Materialia 000 (2019) 100206 Contents lists available at ScienceDirect Materialia journal homepage: www.elsevier.com/locate/mtla Full Length Article Microstructure, indentation and first principles study of AlCuFeMn alloy Amritendu Roy a , M. Ghosh b , H. Gourkar c , P.S. De a,∗ a School of Materials, Metallurgical and Minerals Engineering, IIT Bhubaneswar, India b CSIR-National Metallurgical Laboratory, Jamsedhpur, India c Anton-Paar India Limited, Gurugram, India a r t i c l e i n f o Keywords: Transmission electron microscopy X-ray diffraction Cellular transformation Ordering transformation Spinodal reaction a b s t r a c t Equi-atomic AlCuFeMn alloy with yield strength of ∼1000 MPa and elastic modulus of ∼175 GPa was synthesized using arc-melting. The cast alloy possesses a cellular microstructure with cells exhibiting segregation into Copper and Iron rich regions. Room temperature characterization reveals that the principal phase in Copper rich region is a disordered Body Centered Cubic phase (  3) of threaded morphology in a disordered body centered cubic matrix. The phase morphology is typical of spinodal decomposition with additional ordered face centered cubic nano-precipitates observed. Conversely, the Iron rich region consists of a partially ordered oval Face Centered Cubic phase (   3) in a disordered Body Centered Cubic matrix. The distribution is typical of first order chemical ordering with negligible amount of disordered Face Centered Cubic phase observed. The formation energy of the    3 phase is lower than that of   3 phase and varies with change in atomic position. The overall valence electron concentration of the alloy is 7.25 which results in a combination of body and face centered cubic phases. Such multiple phase formation is an outcome of significant electronic contribution to entropy of mixing for the two principal phases as is reflected from their density of states distribution. 1. Introduction Traditionally, metallic alloys are designed to have one principal element with multiple other elements in small quantity. High entropy (HE) alloys have been proposed [1,2] as a conceptually new class of alloys where (unlike conventional alloys) multiple elements (usually 4 or more) are added in equimolar or near-equimolar concentration. The implication being equimolar multi-component solutions undergoes the highest configurational entropy change, which under negative or zero mixing enthalpy conditions can induce a single disordered phase state [1]. Recent studies have however broken this notion of singularity in phase on account of configurational entropy and brought out the inherent complexities in such alloys. For example, a previously reported face-centered cubic (FCC) CoCrFeNi system is proved to be metastable with precipitation occurring after thermal annealing for 800 h at 750 °C [3]. Further, addition of Al beyond certain limit (i.e. AlCoCrFeNi system) results in multiple phases instead of a single [4]. Recent study on Al 1.3 CoCrCuFeNi alloy by Santodonato et al. [5] observes ordering phenomenon although the configurational entropy (ΔS mix ) change (> 1.73R but < 1.79R) is slightly lower than the maximum achievable theoretical value. The lowest configurational entropy change (∼0.89R) is associated with spinodal transformation of ordered phase into a disordered phase [5] which again is counterintuitive to the original ∗ Corresponding author. E-mail address: parthasarathi.de@iitbbs.ac.in (P.S. De). notion about HE alloys. The above results clearly indicate that enthalpy and entropy change of phases in High Entropy Alloys are interlinked. A thorough evaluation of the same is therefore critical to understand the thermodynamics of multiphase formation. The first principles approach using Density Functional Theory as applied in this work helps to understand this interplay between enthalpy and entropy in details. Any first-principles calculation will require structural symmetry and atomic distribution information over the lattice sites. This necessitates a thorough characterization of phases and their composition followed by Rietveld Refinement analysis to obtain crystal structure details. Appli- cation of this approach to understand lattice strains is already reported in recent HEA literature [6,7]. As will be shown later in this work, Ri- etveld analysis coupled with first principles calculations predicts subtle changes in enthalpy of formation with minute atomic positions changes for the High Entropy Alloy studied. Thus, understanding the effect of atomic distribution on the associated energetics of formation can con- vey important design guideline for these alloys. In this context it is worth noting that currently HE alloy composition selection are selected mostly on empirical or parametric basis where parameters like difference in atomic size () and enthalpy of mixing (ΔH mix ) are used [8]. Small atomic size differences (<4%) and mixing enthalpy (-10 kJ/mole <ΔH mix <5 kJ/mole) combined with high mixing entropy (ΔS mix >13.38 JK -1 mol -1 ) is suggested to stabilize a single https://doi.org/10.1016/j.mtla.2019.100206 Received 4 January 2019; Accepted 12 January 2019 Available online xxx 2589-1529/© 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.